Flow Structure and Dispersion within Algal Mats

International audienceShallow-water estuaries such as Waquoit Bay (Cape Cod, Massachusetts) are characterized by the presence of large algal mats with thicknesses that can represent more than 50% of the total water depth. Flume experiments show these algal mats strongly affect the hydrodynamic circulation by reducing the effective water depth and increasing the friction between the flow and the top of the algal mat. Approximately 90% of the total water flux occurs over the algal mat whereas less than 10% flows through the mat itself. The flow over the algae corresponds to a shear flow, but within the mat it has the characteristics of a pressure-driven flow, with shear restricted to the upper and lower boundaries of the algal mat. Estimated vertical dispersion coefficients within the algal mat vary between 0·03 and 1·6 cm 2 s 1 , and are comparable with or are one order of magnitude larger than those calculated for the shear flow over the algal mat. Experiments also revealed that the observed flow characteristics and dispersion coefficients are relatively insensitive to the density of the algal mat (within normal field ranges), mean flow velocity (up to 10 cm s 1), and/or water depth for the range of parameters used throughout the experiments, indicating that heterogeneities within the mat exert important controls on dispersion within the mat. The introduction of buoyancy forcing due to internally produced heat (as a product of metabolism and/or decomposition of organic matter) or solar heating of the algal mat considerably augments the turbulent kinetic energy; a 1·5 C increase of the water temperature at the base of the algal mat augments the vertical dispersion coefficients by a factor of five